With the rapid development of emerging industries such as new energy vehicles, internet data centers and renewable energy power generation, etc., the power level and voltage level of electrical energy conversion systems are rising day by day. In order to reduce the bulks, weights and costs of power supply systems, the switching frequencies of power electronic devices are rising constantly, thereby significantly reducing the volumes of magnetic elements and other passive elements. However, due to the limitations of semiconductor technologies, devices with higher switching frequencies get lower voltage and current levels. Therefore, combining a plurality of high-frequency power electronic converters in series or parallel has become an effective way to increase the capacity and power density of a power supply system. Besides, combining converters in series or parallel is also an important solution for achieving modular production, debugging and maintenance. The power conversion system formed by combining a plurality of converters in series or parallel is called a multiunit power conversion system in which individual converter is called a power conversion subunit.
In order to equally distribute the total power of a power supply and improve the system reliability, each power conversion subunit of a multiunit power conversion system generally needs to have the same circuit parameters. However, in the actual manufacturing process, it is difficult to make the circuit parameters identical, and there is generally a certain parameter deviation among the subunits. This parameter deviation makes the currents in different subunits unequal, thereby resulting in the local heating of the system, a reduction in efficiency, an increase of output ripples and a reduction of device lifetime, etc. If the frequency of the current deviation among different subunits is much lower than the switching frequency, the subunits will be separately detected and controlled, so that the current sharing among subunits can be realized at the cost of increasing the number of controllers, sensors and connecting wires. However, if the frequency of the current deviation among the subunits is close to the switching frequency, it is difficult to achieve effective current sharing using a control measure. Therefore, it is an effective way to realize the current sharing among subunits by virtue of hardware.
Please refer to FIG. 1, which is a schematic diagram of an example of an existing multiunit power conversion system. As shown in FIG. 1, in the present embodiment, in a multiunit power conversion system formed by a plurality of power conversion subunits 11A connected in parallel, the output terminals of the respective power conversion subunits 11A are connected to inductors L1 which are coupled to each other, and the current of the different power conversion subunits 11A flows into the opposing-polarity terminals of the coupled inductors L1, thereby suppressing the current deviation among the different power conversion subunits 11A. But, in actual use, the multiunit power conversion system still has some shortcomings:
1) In order to reduce the current deviation among the power conversion subunits 11A to a desired value, the coupled inductors L1 must have a sufficiently large inductance value, and therefore it is required that the size of the magnetic core be large enough or the number of turns of winding be sufficient. Moreover, the main circuit current of the power conversion subunits 11A flows through the respective windings of the coupled inductors L1, with a large winding loss and a large wire diameter, particularly in the case of high current applications, so, the coupled inductor L1 has a large package and a big loss.
2) Since the main circuit current flows through the coupled inductors L1 and leakage inductance exists, a great loss of the magnetic core is caused. When the power conversion subunits 11A are connected in series so that there is a high potential difference among the windings of the coupled inductors L1, it is necessary to increase the insulation distance among the windings so that they are not easily coupled tightly, resulting in a further increase in leakage inductance and loss.
3) Due to the influence of different parasitic parameters in each of the power conversion subunits 11A, the switching device drive signal delays to different degrees in the process from being sent by the controller to arriving at the drive terminal of the switching device, so that the current deviation among the power conversion units 11A can be abruptly changed, thereby resulting in an abrupt change of the magnetic induction intensity of the coupled inductors. Therefore, the frequency spectrum of the magnetic flux of the coupled inductor L1 becomes more complex, and the magnetic loss increases.
Please refer to FIG. 2, which is a schematic diagram of another example of the existing multiunit power conversion system. As shown in FIG. 2, in the present embodiment, the multiunit power conversion system is formed by connecting power conversion subunit 11B in parallel, wherein each said power conversion subunit 11B includes an isolated type resonant converter, and the transformer primary or secondary windings of said power conversion subunits 11B are linked to form a star structure of which the center connection point D has a floating potential, thereby suppressing the current deviation among the subunits to a certain extent. But, in actual use, the multiunit power conversion system also has some shortcomings:
1) The method is only applicable to a three-phase parallel structure, and there is a 120° phase shift among the switching device drive signals of the power conversion subunits 11B.
2) Each said power conversion subunit 11B can only be a subunit with a half-bridge structure;
3) The three-phase current balance just relies on the only degree of freedom, that is, the floating potential of the center connection point, which has a limited control on the current of said the respective power conversion subunits 11B. If the number of parallel power conversion subunits 11B exceeds three, the current-sharing effect will be further deteriorated. Therefore, this method has a limited effect of three-phase current sharing.